Exemplary methods hitherto known for production of ultrafine metal oxide particles having diameters of not greater than 1 .mu.m include hydrolysis of a metal alkoxide and utilization of an interface of liquid and plasma generated by electrolysis as disclosed in JP-B-6-86285.
Exemplary methods hitherto known for production of ultrafine metal hydroxide particles having diameters of not greater than 1 .mu.m include precipitation such as co-precipitation and hydrolysis of a metal alkoxide.
In the method involving the hydrolysis of a metal alkoxide, ultrafine oxide particles are obtained by dissolving the metal alkoxide as a starting material in a large amount of alcohol, then adding water and a catalyst to the resulting solution, and subjecting the alkoxide to hydrolysis and subsequent polycondensation.
In the method utilizing an interface of liquid and plasma generated by electrolysis, ultrafine particles including oxide particles are obtained by reacting ions leaching out of an anode material with oxygen ions in the interface of the liquid and plasma generated by electrical breakdown of a gas occurring on a cathode.
In the method of producing ultrafine metal hydroxide particles through co-precipitation, the ultrafine metal hydroxide particles are precipitated by changing the pH of a solution containing metal ions.
The aforesaid methods are successful in production of ultrafine particles of a metal oxide or a metal hydroxide, but have a difficulty in low-cost production.
More specifically, it is necessary, in the method involving the hydrolysis of a metal alkoxide, to keep the concentration of the starting material in the solution at a low level during the production of the ultrafine particles for prevention of aggregation of the ultrafine particles. Therefore, a large amount of an alcohol solvent is used, requiring a larger scale production plant. Further, a relatively expensive alkoxide is used as the starting material. Accordingly, the production cost of the ultrafine particles is high.
The method utilizing the plasma-liquid interface involves generation of plasma by electrolysis, resulting in a higher power consumption. Accordingly, the production cost of the ultrafine particles is high.
In the method of co-precipitation, the concentration of metal ions contained in the solution as the starting material of the ultrafine particles is required to be kept at a low level in order to prevent simultaneous development of the core formation and the aggregation of the ultrafine particles. However, this is very difficult to control.
Further, well known methods for producing ultrafine particles of magnetite (Fe.sub.3 O.sub.4) include reduction of goethite (.alpha.-FeOOH) and oxidation of a ferrous salt by addition of an alkali.
JP-A-4-238819 discloses a method of producing ultrafine magnetite particles having particle diameters of 10 to 100 nm, in which an alkali is added to an aqueous solution containing ferrous ions for production of ferrous hydroxide while an inert gas is blown in the aqueous solution for reduction of the concentration of oxygen dissolved therein, and the resulting dispersion is heated up to 60 to 100.degree. C.
However, the magnetite particles produced through the reduction of goethite (.alpha.-FeOOH) or through the oxidation of a ferrous salt by addition of an alkali have a relatively large average diameter, i.e., not smaller than 100 nm. The method disclosed in JP-A-4-238819 involves complicated process steps with a difficulty in particle size control for uniformity in particle size and, therefore, are not advantageous in terms of industrial-scale production.